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A > Cs) and dissolved (loading equal to or less than saturation, A < Cs). For the latter, solutions to Fick's second law (Eq. 4.2) are well known, and the particular expression for semi-infinite geometry is

This is exactly the same result from Eq. (4.10) in the limit of A ^ Cs.

In order to achieve near-zero-order release from the matrix, a unique geometry, a specific nonuniform initial concentration profile, and/or a combined diffusion/erosion/swelling mechanism provide theoretical basis for such an approach.

4.1.3 Diffusion across a barrier membrane

For diffusion through a homogeneous membrane (thickness h) that is sandwiched between external media, an infinite reservoir frequently is assumed in the donor side and a perfect sink in the receiver side. There are two different initial conditions, which give different initial diffusion rates of either lower or higher than steady-state values.

The first "initial condition" is no presence of drug in the membrane, corresponding to a delivery system that is used immediately after manufacture. Mathematically, this problem is stated as Fick's second law (Eq. 4.2) with the following boundary and initial conditions: